Electron microscopes explained

Optical microscopes are useful, versatile and inexpensive, but they are inherently limited by physics.

Visible light has a set range of wavelengths. And the wavelengths of optical microscopes light are too long to resolve extremely small objects.

Essentially, if an object is smaller than half the wavelength of light, you won’t be able to use light to see it. You need something with an even shorter wavelength.

Enter the electron microscope.

Developed to overcome the optical limits of light, electron microscopes fire a stream of electrons at a sample instead. Electrons have a much smaller wavelength than light so they’re able to visualise objects down to a fraction of a nanometre in size.

Scanning Electron Microscope - SEM

How they work

Electron microscopes work in a surprisingly similar fashion to regular optical microscopes. There are several types, but they share the same basic functions.

A stream of high voltage electrons is formed by the electron source (usually heated tungsten). These super-charged electrons are then focused and directed by a series of electromagnetic coils, similar to the focusing lenses and condenser of an optical microscope.

The electrons pass through or bounce off the sample (depending on the type of microscope). They are then registered and detected by the equipment, which translates the readings into a very high-resolution image.

Dust Mite under a SEM

Advantages

Their obvious advantage is incredibly high resolution and magnification – super-clear images of specimens and structures that could never be seen otherwise.

Electron microscopes are also very versatile – they have applications in a wide range of fields and are capable of observing organic and inorganic samples.

Disadvantages

While electron microscopy has led to breakthrough science, it has some drawbacks.   

Electron microscopes are large, cumbersome, and expensive (from US$50,000 up to $30 million). They require constant maintenance and can be very sensitive to magnetic fields and vibrations.

Another limitation is that they can’t be used to observe living specimens. The microscope interior is kept in a vacuum, which tends to be detrimental to biological processes.

Koil Bacteria under a SEM

There can also be some problems with the image – they’re always black and white and can suffer from artifacts (damage) if the sample isn’t prepared properly.

Types

There are a few different types of electron microscope. Two of the most common are transmission electron microscopes (TEM) and scanning electron microscopes (SEM).

Transmission electron microscope

TEMs are quite similar to compound microscopes – electrons are fired through a very, very thin slice of a sample and are picked up by a fluorescent screen, where the image is displayed or recorded.

Transmission Electron Microscope - TEM

The process takes only a few seconds and produces a 2D black-and-white image.

This was the first form of electron microscope developed back in 1931. Early models had some issues with optical aberrations, but these have since been corrected.